In time division duplex (TDD) systems, the same frequency interval may be used for both uplink and downlink transmission, wherein the uplink and downlink communication are separated in time. One example of a TDD system that will be used herein for illustrative, although not limiting, purposes is the TDD version of E-UTRA LTE (Evolved Universal Terrestrial Radio Access, Long term Evolution—advocated by 3GPP; the Third Generation Partnership Program), hereinafter LTE TDD. However, it should be noted that the problems and embodiments described herein may be equally applicable in any TDD system.
Switching between uplink and downlink (and vice versa), which may occur relatively often (e.g. one or more times during a frame in LTE TDD), may pose some problems due to, for example, propagation delay as will be illustrated in connection with FIG. 1A for a LTE TDD situation.
FIG. 1A schematically illustrates timing of uplink (UL) and downlink (DL) communication at a first network node 110 and at a first wireless communication device 100 communicating with the first network node 110. Also shown is the timing of uplink (UL) and downlink (DL) communication at a second, neighboring, network node 120 and at a second wireless communication device 130 communicating with the second network node 120.
Due to propagation delay between the network nodes and respective wireless communication devices a downlink packet transmitted by a network node arrives somewhat later at the respective wireless communication device (see, for example, downlink transmissions 111, 115, 121, 125 and corresponding downlink receptions 101, 105, 131, 135) and correspondingly for an uplink packet transmitted by a wireless communication device (see, for example, uplink transmissions 103, 104, 133, 134 and corresponding uplink receptions 113, 114, 123, 124).
To make each uplink and downlink packet fit the subframe structure (illustrated by dashed vertical lines in FIG. 1A) at each network node, a timing advance, TA, 106, 136 is applied at each wireless communication device for advancing transmission of uplink packets in relation to the timing of reception of downlink packets. Typically, the network node informs the wireless communication device of which timing advance is to be used, and the timing advance corresponds to (at least) twice the propagation delay between a network node and corresponding wireless communication device to avoid overlap between uplink and downlink packets at the network node. The terms timing advance and time advance will be used interchangeably herein. The general concept of a timing advance is well known in the art and will not be elaborated on further herein. Further details may be found, for example, in “3G Evolution: HSPA and LTE for Mobile Broadband” (Chapter 16.3.5 Uplink timing advance) by Erik Dahlman, Stefan Parkvall, Johan Sköld, and Per Beming, 2007 (ISBN 978-0-12-372533-2).
To avoid overlap between downlink and uplink packets at the wireless communication device a buffer subframe 112 may be used when switching from downlink to uplink communication. In LTE TDD the buffer subframe is termed special subframe (SSF) as opposed to standard subframes (SF). As illustrated in FIG. 1A, the special subframe comprises three parts: first a downlink pilot time slot (DwPTS), in the middle a guard period (GP), and finally an uplink pilot time slot (UpPTS). The guard period should be long enough to ensure that there is no overlap between the DwPTS and the UpPTS at the wireless communication device as illustrated at 109. Generally, the buffer subframe should be constructed to avoid overlap between downlink and uplink communication at the wireless communication device, and is thereby associated with the timing advance since the timing advance and the buffer subframe together define the time between downlink and uplink communication at the wireless communication device. Since the timing advance is typically (essentially) proportional to the propagation delay, large cells typically require a longer guard period than small cells.
Generally, the length of the guard period of the buffer subframe and the length of the timing advance may depend on other factors as well. For example, the necessary time for circuitry in the network node and wireless communication device, respectively, to switch between uplink and downlink communication (and vice versa) may be taken into account. However, circuitry switching is typically relatively fast compared to other components such as propagation delay.
Another example of factors that may affect the length of the guard period and the length of the timing advance is interference between cells (e.g. from neighboring network nodes or from wireless communication devices communicating with neighboring network nodes). Such interference between downlink and uplink communication may originate from the fact that the propagation delay is typically longer for signals from neighboring cells than for signals from the own cell.
Alternatively, or additionally, such interference may arise if the network nodes are not fully synchronized in time. This is illustrated in FIG. 1A where a cell synchronization error 117 between network nodes 110 and 120 results in an overlap (a collision) 118, 128 between downlink and uplink communication at the respective network nodes. Thus, 128 illustrates that when network node 120 is to receive UL packet 124 it will be interfered by transmission by the network node 110 of DL packet 115. Similarly, 118 illustrates that when DL packet 115 transmitted by network node 110 is to be received as DL packet 105, it may be interfered by UL packet 124 transmitted by wireless communication device 130 as UL packet 134.
To avoid this type of interference, TDD systems typically specify tight synchronization between network nodes. For example, 3GPP Technical Specification (TS) 36.133 (v. 11.6.0 (2013-09), 3rd Generation Partnership Project; Evolved Universal Terrestrial Radio Access (E-UTRA); Requirements for support of radio resource management) specifies cell phase synchronization requirements for LTE TDD (see e.g. tables 7.4.2-1 and 7.4.2-2). Examples of how tight synchronization may be achieved include using a GNSS (global navigation satellite system) receiver to derive accurate timing from satellite signals and/or using synchronization protocols (e.g. Precision Time Protocol, PTP, IEEE 1588) to distribute time from an accurate time source.
However, in some situations it may be desirable to compromise the tight synchronization requirements. For example, for indoor communication systems it may be unnecessarily costly to deploy either of the GNSS solution and the synchronization protocol solution. In fact, these solutions may, sometimes, be impossible to implement due to, for example, restrictions related to the building. Thus, it may be desired to use a loosely synchronized system, for example, with Network Time Protocol (NTP) which implicates lower synchronization accuracy.
Therefore, there is a need for approaches to avoid, or at least reduce, interference between cells when neighboring network nodes are not tightly synchronized in time.